1. Field of the Invention
The present invention relates to a catadioptric system suitable for observing an enlarged optical image of an object, and to an image pickup apparatus including the catadioptric system.
2. Description of the Related Art
Conventional pathology inspections use an optical microscope through which a pathology specimen (sample) is observed directly by person's eyes. On the other hand, recent pathology inspections use a virtual microscope capturing image data of the pathology specimen to allow observation of the image data displayed on a monitor. Such a virtual microscope enables plural persons to simultaneously observe the image data of the pathology specimen displayed on the monitor. Moreover, the use of the virtual microscope provides many advantages such as asking a distant pathologist for diagnosis using shared image data. However, the use of the virtual microscope has a problem that acquisition of the image data of the pathology specimen through image capturing requires a long time.
One of reasons for requiring such a long time is necessity to acquire the image data of the pathology specimen having a wide area through a narrow image capturing field. Such a narrow image capturing field makes it necessary to produce one large image by joining images acquired by plural image capturing operations with scanning. Therefore, in order to reduce number of times of the image capturing operations to shorten the time required for acquiring the image data, an optical system (image capturing optical system) providing a wide image capturing field is required.
Besides such an optical system providing the wide image capturing area for observation of the pathology specimen, an optical system having high resolution in a visible wavelength range (that is, in a wide wavelength range) is desired. Thus, Japanese Published Examined Patent Application No. 60-034737 discloses a microscope objective lens constituted by a refractive optical system and having well-reduced aberrations in an entire visible wavelength range, which is suitable for observation of specimens such as biological cells.
Moreover, Japanese Patent Laid-Open No. 2007-514179 discloses a super-wideband ultraviolet microscope system using a catadioptric imaging system and having high resolving power over a wide wavelength range, which is used for inspecting defects existing in integrated circuits and photomasks. Furthermore, WIPO Patent Application WO00/039623 discloses a catadioptric imaging system suitable for exposing minute patterns to a wide area to manufacture semiconductor devices. In addition, Japanese Patent Laid-Open No. 2009-063655 discloses a microscope system that efficiently acquires wide-screen digital image data to be used for observation of pathology specimens such as biological cells.
Image capturing optical systems for the virtual microscopes are generally required to have high optical performance in a wide field of view, which is achieved by good correction of various aberrations such as spherical aberration, comatic aberration and astigmatism.
On the other hand, suppression of negative influences (such as distortion) caused by positional errors of the pathology specimen and an image sensor requires that the image capturing optical system have telecentricity on both object and image sides. In particular, wide-screen image capturing requires good telecentricity over an entire observation field of view. Moreover, RGB (red, green and blue) color image capturing for acquiring a color image requires good telecentricity over an entire image capturing wavelength range.
The microscope objective lens disclosed in Japanese Published Examined Patent Application No. 60-034737 sufficiently reduces various aberrations over the entire visible wavelength range, but does not necessarily provide an observation field of view having a sufficient size. Also, the wideband microscope catadioptric imaging system disclosed in Japanese Patent Laid-Open No. 2007-514179 sufficiently reduces aberrations over a wide wavelength range and has high resolving power, but does not necessarily provide an observation field of view having a sufficient size.
Furthermore, the catadioptric imaging system disclosed in WIPO Patent Application WO00/039623 has high resolving power over a wide wavelength range, but does not necessarily corrects aberrations in a sufficiently wide wavelength range.
Additionally, the microscope system disclosed in Japanese Patent Laid-Open No. 2009-063655 performs image capturing by using plural image sensors arranged in an image capturing field of an objective lens and efficiently acquires wide-screen image data by joining data of captured images. The microscope system provides overlapping portions to the respective captured image data to enable accurate joining of mutually adjacent captured image data.
However, if the objective lens in the microscope system has low telecentricity on its image side, displacement of positions of the image sensors in a direction of an optical axis due to manufacturing errors or focus adjustment generates distortion. Larger distortion causes a necessity of providing wider overlapping portions for joining the mutually adjacent captured image data. Moreover, when performing image capturing for each of RGB to acquire a color image, it is necessary to provide sufficient overlapping portions so as to also enable joining of captured image data of an image capturing wavelength at which the image side telecentricity becomes lowest.
In such a case, number of pixels to be used only as the overlapping portions for joining of the image data is increased with respect to the wide-screen image data to be acquired, which increases volume of data and thereby makes it difficult to acquire the wide-screen image data at high speed.
In order to provide a wide observation field of view and high optical performance over a wide wavelength range for observing an enlarged image of a specimen, use of an optical surface having an aspheric shape is effective.
However, even if simply using such an aspheric optical surface, it is difficult to correct various aberrations well over a wide wavelength range and to achieve high optical performance over a wide observation field of view.
In particular, in order to realize a catadioptric system having good image side telecentricity in the wide wavelength range and in the wide observation field of view by using an aspheric optical surface, it is important to provide an optical surface having an appropriate aspheric shape to an appropriate position in the catadioptric system.